Method for Controlled Shutdown of an Implantable Medical Device

ABSTRACT

An improved implantable pulse generator (IPG) containing graceful shutdown circuitry is disclosed. A magnet sensor senses the presence of an emergency shutdown magnet. Output of the magnet sensor is conditioned by a signal conditioning circuit. Output of the signal conditioning circuit is delayed by a delay element before being fed to a power cut-off switch, which cuts-off power to the IPG circuitry. An interrupt signal is routed from before the delay element to the IPG processor as an indicator of imminent shutdown. The processor launches shutdown routine that carries out shutdown operations such as logging the emergency shutdown event, saving and closing open files, saving data from volatile memory to non-volatile memory, etc., before the power cut-off switch is activated upon elapsing of delay provided by the delay element. The magnet sensor, signal conditioning circuit, and delay element are powered separately from the rest of the circuitry of the IPG.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/030,848, filed Feb. 18, 2011, which was a non-provisional ofU.S. Provisional Patent Application Ser. No. 61/318,198, filed Mar. 26,2010. Priority is claimed to both applications, and both areincorporated herein by reference in their entireties.

FIELD OF THE INVENTION

The present invention relates to improved emergency shutdown circuitryfor an implantable medical device.

BACKGROUND

Implantable stimulation devices are devices that generate and deliverelectrical stimuli to body nerves and tissues for the therapy of variousbiological disorders, such as pacemakers to treat cardiac arrhythmia,defibrillators to treat cardiac fibrillation, cochlear stimulators totreat deafness, retinal stimulators to treat blindness, musclestimulators to produce coordinated limb movement, spinal cordstimulators to treat chronic pain, cortical and deep brain stimulatorsto treat motor and psychological disorders, and other neural stimulatorsto treat urinary incontinence, sleep apnea, shoulder subluxation, etc.The description that follows will generally focus on the use of theinvention within a Spinal Cord Stimulation (SCS) system, such as thatdisclosed in U.S. Pat. No. 6,516,227. However, the present invention mayfind applicability in any implantable medical device system.

As shown in FIGS. 1A and 1B, a SCS system typically includes anImplantable Pulse Generator (IPG) 100, which includes a biocompatibledevice case 30 formed of a conductive material such as titanium forexample. The case 30 typically holds the circuitry and battery 26necessary for the IPG to function, although IPGs can also be powered viaexternal RF energy and without a battery. The IPG 100 is coupled toelectrodes 106 via one or more electrode leads (two such leads 102 and104 are shown), such that the electrodes 106 form an electrode array110. The electrodes 106 are carried on a flexible body 108, which alsohouses the individual signal wires 112 and 114 coupled to eachelectrode. In the illustrated embodiment, there are eight electrodes onlead 102, labeled E₁-E₈, and eight electrodes on lead 104, labeledE₉-E₁₆, although the number of leads and electrodes is applicationspecific and therefore can vary. The leads 102, 104 couple to the IPG100 using lead connectors 38 a and 38 b, which are fixed in anon-conductive header material 36, which can comprise an epoxy forexample.

As shown in FIG. 2, the IPG 100 typically includes an electronicsubstrate assembly 14 including a printed circuit board (PCB) 16, alongwith various electronic components 20, such as microprocessors,integrated circuits, and capacitors mounted to the PCB 16. Two coils(more generally, antennas) are generally present in the IPG 100: atelemetry coil 13 used to transmit/receive data to/from an externalcontroller 12; and a charging coil 18 for charging or recharging theIPG's battery 26 using an external charger 50. The telemetry coil 13 istypically mounted within the header 36 of the IPG 100 as shown, and maybe wrapped around a ferrite core 13′.

As just noted, an external controller 12, such as a hand-held programmeror a clinician's programmer, is used to wirelessly send data to andreceive data from the IPG 100. For example, the external controller 12can send programming data to the IPG 100 to dictate the therapy the IPG100 will provide to the patient. Also, the external controller 12 canact as a receiver of data from the IPG 100, such as various datareporting on the IPG's status. The external controller 12, like the IPG100, also contains a PCB 70 on which electronic components 72 are placedto control operation of the external controller 12. A user interface 74similar to that used for a computer, cell phone, or other hand heldelectronic device, and including touchable buttons and a display forexample, allows a patient or clinician to operate the externalcontroller 12. The communication of data to and from the externalcontroller 12 is enabled by a coil (antenna) 17.

The external charger 50, also typically a hand-held device, is used towirelessly convey power to the IPG 100, which power can be used torecharge the IPG's battery 26. The transfer of power from the externalcharger 50 is enabled by a coil (antenna) 17′. For the purpose of thebasic explanation here, the external charger 50 is depicted as having asimilar construction to the external controller 12, but in reality theywill differ in accordance with their functionalities as one skilled inthe art will appreciate.

Wireless data telemetry and power transfer between the external devices12 and 50 and the IPG 100 takes place via inductive coupling, andspecifically magnetic inductive coupling. To implement suchfunctionality, both the IPG 100 and the external devices 12 and 50 havecoils which act together as a pair. In case of the external controller12, the relevant pair of coils comprises coil 17 from the controller andcoil 13 from the IPG 100. In case of the external charger 50, therelevant pair of coils comprises coil 17′ from the charger and coil 18from the IPG 100.

When data is to be sent from the external controller 12 to the IPG 100for example, coil 17 is energized with an alternating current (AC). Suchenergizing of the coil 17 to transfer data can occur using a FrequencyShift Keying (FSK) protocol for example, such as disclosed in U.S.Patent Publication 2009/0024179. Energizing the coil 17 produces amagnetic field, which in turn induces a voltage in the IPG's coil 13,which produces a corresponding current signal when provided a closedloop path. This voltage and/or current signal can then be demodulated torecover the original data. Transmitting data from the IPG 100 to theexternal controller 12 occurs in essentially the same manner.

When power is to be transmitted from the external charger 50 to the IPG100, coil 17′ is again energized with an alternating current. Suchenergizing is generally of a constant frequency, and may be of a largermagnitude than that used during the transfer of data, but otherwise thebasic physics involved are similar.

The IPG 100 can also communicate data back to the external charger 50 bymodulating the impedance of the charging coil 18. This change inimpedance is reflected back to coil 17′ in the external charger 50,which demodulates the reflection to recover the transmitted data. Thismeans of transmitting data from the IPG 100 to the external charger 50is known as Load Shift Keying (LSK), and is useful to communicate datarelevant during charging of the battery 26 in the IPG 100, such as thecapacity of the battery, whether charging is complete and the externalcharger can cease, and other pertinent charging variables. LSKcommunication from an IPG 100 to an external charger is discussedfurther in U.S. Patent Publication 2010/0179618.

As is well known, inductive transmission of data or power can occurtranscutaneously, i.e., through the patient's tissue 25, making itparticularly useful in a medical implantable device system. During thetransmission of data or power, the coils 17 and 13, or 17′ and 18,preferably lie in planes that are parallel, along collinear axes, andwith the coils as close as possible to each other. Such an orientationbetween the coils 17 and 13 will generally improve the coupling betweenthem, but deviation from ideal orientations can still result in suitablyreliable data or power transfer.

IPG 100 can comprise circuitry that enables a user or a clinician toshutdown the IPG 100 in case of emergencies. Such emergencies can arisewhen the IPG 100 malfunctions, undesirably over-stimulates the patient,does not provide stimulation at all, etc. FIG. 3 shows circuitry 302that is traditionally used in the IPG 100 for emergency shutdown. A useror clinician brings a magnet 300 near the location on the patient's bodywhere the IPG 100 is situated. A magnet sensor 306, such as a reedswitch, detects the presence of magnet 300 by way of sensing itsmagnetic field, and sends an electrical signal (voltage or current) to asignal conditioning circuit 308. The signal conditioning circuit 308suppresses any stray and transient signals (e.g., reed bounce) receivedfrom the magnet sensor 306. Once a sufficient signal indicating presenceof magnet 300 is detected, the signal conditioning circuit 308 outputs asignal that opens switch 310. Once switch 310 is open, Rest of theDevice (ROD) 312 will be disconnected from battery 26. ROD 312 willtypically include all the circuitry responsible for the functioning ofthe IPG 100. For example, ROD 312 can include the microprocessor,charging circuits, telemetry circuits, stimulation circuits, volatileand non-volatile memory, etc. Upon being disconnected from the battery,these circuits will cease to function.

Although the aim of an emergency stop may be to immediately halt anystimulation being received by the IPG 100, an abrupt shut down like theone depicted in FIG. 3, can have certain disadvantages. For example, anydata stored in volatile memory (e.g., RAM) will be lost, which data mayinclude current stimulation program parameters. If the currentstimulation parameters were intended to be stored/saved in non-volatilememory (into a stimulation parameter file, for example), an abrupt powerloss may prevent the microprocessor from completing the save operation.In other instances it is also possible that the microprocessor is in theprocess of moving data from volatile memory to non-volatile memory whenpower loss occurs. This may result in only a partial data store—possiblycorrupting the data stored in non-volatile memory.

In other instances it may be beneficial, from an analysis point of view,to record data relevant to the emergency shutdown itself. However, anabrupt shutdown may not allow the microprocessor to log this informationinto non-volatile memory.

A solution to this problem is provided in this disclosure in the form ofan improved emergency shutdown circuit for an IPG 100 or otherimplantable medical device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show an implantable medical device, and the manner inwhich an electrode array is coupled to the IPG in accordance with theprior art.

FIG. 2 shows the relation between the implantable medical device, anexternal controller, and an external charger.

FIG. 3 shows a traditional emergency IPG shutdown circuit in an IPG.

FIG. 4 shows a first embodiment of improved emergency shutdown circuitryfor the IPG of FIG. 3 to provide graceful shutdown.

FIG. 5 shows a detailed exemplary circuit diagram of the emergencyshutdown circuitry of FIG. 4.

FIGS. 6A-6D show timing diagrams of various signals of the circuitsshown in FIGS. 4 and 5.

FIG. 7 shows a flowchart depicting shutdown operations performed by theIPG.

DETAILED DESCRIPTION

The description that follows relates to use of the invention within aspinal cord stimulation (SCS) system. However, it is to be understoodthat the invention is not so limited, and could be used with any type ofimplantable medical device system.

The inventor addresses the problem of data loss during emergencyshutdown of the IPG 100 by including a delay element 412 between thesignal conditioner 308 and the switch 310 in an improved emergencyshutdown circuit 402, as shown in FIG. 4. In contrast with emergencyshutdown circuit 302 of FIG. 3, where the output of the signalconditioning circuit 308 is fed directly to the power cut-off switch310, the emergency shutdown circuit 402 in FIG. 4 instead delays theoutput of the signal conditioning circuit 308 before it is fed to thepower cut-off switch 310. In addition, output of the signal conditioner308 is provided as an interrupt signal 414 to the ROD 312. As a result,as soon as the signal conditioning circuit 308 outputs signal indicatingthat magnet 300 has been detected, ROD 312 receives a notification, inthe form of the interrupt signal 414, of an imminent emergency shutdown.Interrupt signal 414 is fed to an interrupt pin of processor 418, whichcan immediately launch a shutdown routine (discussed further below) thatallows it to carry out shutdown operations such as logging the emergencyshutdown event, saving and closing open files, etc. A timing delay(T_(d)) produced by delay element 412 is selected to provide theprocessor 418 enough time to complete the launched shutdown routine,which time can vary between applications.

After the elapse of grace-period delay T_(d), the delay element 412outputs signal 416. Signal 416 is fed to the input of shutdown switch310 and a reset pin RST of the processor 418. Upon receiving signal 416,switch 310 (which is normally closed) opens and, as a result,disconnects ROD 312 from battery 26. The emergency shutdown circuit 402,comprising magnet sensor 306, signal conditioning circuit 308, and thedelay element 412, is powered separately from the ROD 312. Furthermore,emergency shutdown circuit 402 may be implemented on a separatesubstrate than the one on which ROD 312 is implemented. This reducessystem faults that may occur in the ROD 312 from affecting the emergencyshutdown circuit 402.

FIG. 5 shows a detailed exemplary circuit diagram of emergency shutdowncircuit 402. In default state—when no magnetic field is sensed by themagnet sensor 306—the output of the magnet sensor is pulled low by thepull down resistor R_(pull down) 521. Therefore, the output 414 of thesignal conditioning circuit 308 is low. Note that the output 414 of thesignal conditioning circuit 308 is coupled to an interrupt input of theprocessor (418, FIG. 4). It is assumed that the processor 418 activatesan interrupt routine only when the interrupt input of the processor isat logic high value. Therefore, in the default state, the processor isnot interrupted. Furthermore, by virtue of inverters 531 and 534, theoutput 416 of the delay element 412 is also logic low and the capacitor533 is fully charged. Therefore, shutdown switch 310 remains closed,keeping ROD 312 connected to battery 26. In other words, without thedetection of an emergency stop by way of magnet 300, ROD 312 continuesto receive power.

As stated before, when an emergency shutdown is needed, magnet 300 isbrought near the IPG 100. Magnet sensor 306 senses the approach ofmagnet 300. Magnet 300 is typically a permanent magnet that produces amagnetic field that is considerably larger than the earth's magneticfield (˜0.5 gauss). Magnet sensor 306 is preferably designed such thatit produces a null output when placed in only the earth's magneticfield, but produces an output when placed in the magnetic field producedby magnet 300. Several types of magnetic field sensors 306 can be used,for example, reed switches, InSb magnetoresistors, Hall devices, GMRsensors, etc.

As an example, FIG. 5 shows a reed switch 512 being used as magnetsensor 306. One end of the reed switch 512 is connected to the supplyvoltage V_(bat), while the other end is connected to pull down resistorR_(pull down) 521. The pull down resistor R_(pull down) 521 maintainsthe voltage at the output of the magnet sensor 306 to a voltage that isapproximately equal to GND. The reed switch 512 includes a pair offlexible, ferromagnetic contacts, which get magnetized by the presenceof the magnetic field of magnet 300. This magnetization causes thecontacts to attract each other and close the circuit. This results inthe output of the magnet sensor 306 to be pulled high.

It is not unusual for the output of the magnetic sensor 306 to toggle orfluctuate between high and low values. This can be caused due to reedbounce within the reed switch 512, or by erratic movement of the magnet300. Fluctuations can also be caused by magnets other than magnet 300,such as magnets in DC motors and other electrical devices that thepatient may commonly encounter in close proximity around the house orworkplace. It is not desired that such fluctuations be interpreted as anactual emergency shutdown condition. Therefore, a low pass filter,formed by resistor R_(f) 522 and capacitor C_(f) 523, within the signalconditioning circuit 308 is connected to the output of the magnet sensor306, which low pass filter filters the output of the magnet sensor 306before it is fed to inverter 525. The frequency response of the low passfilter can be adjusted by the values of R_(f) and C_(f). Although apassive filter formed by resistor R_(f) 522 and capacitor C_(f) 523 hasbeen shown in FIG. 5, a person skilled in the art will appreciate thatactive filters designed using components such as transistors,operational amplifiers, etc., can also be used.

Inverter 525, Schmitt trigger 526, and buffer 527 digitize the filteredoutput of the magnet sensor 306. Once the filtered output of the magnetsensor 306 reaches a sufficiently high value, the output of the inverter525 goes low. The input to inverter 525, i.e., the output of magnetsensor 306, may transition from low to high relatively slowly by virtueof the passive filter. This can cause excessive current draw through theinverter 525, which is typically (but not necessarily) implemented usingCMOS technology. In particular, the “crowbar current” experienced duringa CMOS logic state transition can be exacerbated by the filter. To limitthis crowbar current, a current source 524 is placed in series with thesupply voltage (V_(bat)) of the inverter 525.

Output of the inverter 525 is fed to an inverting Schmitt trigger 526,which will quickly transition from low to high even if the output of theinverter 525 changes from high to low relatively slowly. The output ofthe Schmitt trigger 526 is fed to the input of buffer 527, which followsthe input to the trigger 526. The operation of signal condition circuit308 can be seen in FIGS. 6A and 6B, which respectively show theunfiltered output of the magnet sensor 306, and the digitized output ofthe signal condition circuit 308. Time T_(sc) denotes the time takenfrom the moment the magnet 300 is first sensed by the magnet sensor 306to the moment the output of signal condition circuit 308 goes high, andessentially represents the time taken to appropriate condition thesignal. T_(sc) would normally be significantly less than thegrace-period delay T_(d), and may be included as part of the graceperiod delay.

Output of signal conditioning circuit 308 is fed to two locations—as aninterrupt signal 414 to processor 418 and to the delay element 412. Theinterrupt 414 to the processor 418 starts an interrupt service routinewithin the processor 418 that carries out controlled shutdown of the IPG100. The delay element 412 delays the output from the signalconditioning circuit 308 from opening switch 310 by an amount T_(d),which is designed to provide the processor 418 sufficient time to carryout the required shutdown operations. The shutdown operation of theprocessor 418 is described with further detail below with respect toFIG. 7.

The delay element 412 includes inverter 531, delay capacitor 533, andinverter 534. When input of inverter 531, which is connected to theoutput of signal conditioning circuit 308, changes from low to high, theoutput of inverter 531, changes from high to low. Note that in defaultstate (with no emergency shutdown) the delay capacitor 533 is fullycharged to a high state. Therefore, the charge stored in the delaycapacitor is drained to ground GND via the n-MOS transistor (not shown)of inverter 531 and current source 532. The amount of time it takes forthe output of the inverter 531 to go from high to low depends upon theamount of time it takes to discharge delay capacitor 533, which timedepends upon the size of delay capacitor 533, the size of the n-MOStransistor of the inverter 531 and the value of the current source 532.Having a small size n-MOS transistor and a small value of current source532 can increase the time it takes to discharge capacitor. Dischargetime can also be increased by increasing the size of the delay capacitor533. When the output of the inverter 531 begins to change from high tolow, the output of the inverter 534 begins to change from low to high.In case of inverter 534, capacitance at the output of the inverter 534is charged via the p-MOS transistor (not shown) of the inverter 534 andcurrent source 535. Capacitance at the output of inverter 534 willpredominantly be interconnect capacitance. The amount of time requiredto charge this interconnect capacitance depends upon the value of thecurrent source 535 and the size of the p-MOS transistor of inverter 534.Decreasing the value of the current source 535 can increase thedischarge time. Similarly, decreasing the size of the p-MOS transistorwill increase the discharge time. In any event, these various values setthe total delay T_(d) offered by the delay element 412. After delayT_(d), the output of the delay element 412 goes high as shown in FIG.6C. At this time, the shutdown switch 310 is opened, disconnecting theROD 312 from the battery 26.

As stated earlier, output of signal conditioning circuit 308 is fed asan interrupt signal 414 to processor 418. Processor 418 may have one ormore interrupt pins. Preferably, the interrupt signal 414 is connectedto a non-maskable interrupt (NMI) pin of the processor 418 so that theinterrupt signal 414 is never ignored. Upon receiving an interrupt, theprocessor 418 launches an interrupt service routine (ISR), such as ashutdown routine, associated with that interrupt pin, which ISR isillustrated in FIG. 7. This ISR carries out shutdown operations such aslogging the emergency shutdown event, saving and closing open files,etc., before time T_(d) has lapsed, i.e., before the ROD 312 isdisconnected from its power supply.

In step 702, processor 418 refreshes a watch-dog timer 422 (FIG. 4). Thewatch-dog timer 422 resets the processor 418 upon completion of apredetermined time interval, and can be implanted as a counter. Forexample, a down counter with initial count of FFFF will reset theprocessor 418 when it reaches 0000. Processor 418 periodically refreshesthe watch-dog timer by resetting the counter before it reaches 0000.Non-servicing of the watch-dog timer before it reaches 0000 can serve asan indication of a fault condition or state. Therefore, the resetting ofthe processor 418 by the watch-dog timer 422 can bring the processorback to normal operation. In step 702, the processor resets thewatch-dog timer 422 right at the outset of the shutdown routine so thatthe processor 418 can dedicate maximum available time in carrying outshutdown operations. Note that although the watch-dog timer 422 in FIG.4 is shown as a separate entity from the processor 418, it is notuncommon for processors 418 to include a watch-dog timer 422.

In step 704 processor 418 stops any stimulation given to the patient,i.e. ceases any therapeutic operation being performed by the implantablemedical device. Alternatively, operation of the implantable medicaldevice may not cease entirely, but instead may enter a different modesafer for the patient. For example, in a neurostimulator, initiation ofthe shutdown routine may simply reduce the magnitude of the stimulationsettings to levels known to be conservatively safe for even the mostsensitive patient.

In step 706, processor 418 logs the emergency shutdown event in a logfile stored in non-volatile memory 420 (FIG. 4). This includes variousstatus data which can be telemetered back to the external controller 12for further analysis. Such status data can include operation parameterssuch as current battery voltage, V_(bat), stimulation status data, timestamps, including the time when the emergency shutdown event occurred,etc. The time stamps can be derived from an internal IPG clock (notshown). In some cases, the internal IPG clock may be inaccurate,although the technique disclosed in U.S. Patent Publication 2010/0125316can help in this regard.

In step 708, processor 418 stops and closes any open or active resourcessuch as file systems. This can entail closing log files, stimulationfiles that contain stimulation data, or system files required for theoperation of the processor 418. By systematically closing open or activefiles, corruption or loss of data associated with an abrupt shutdown canbe reduced or avoided. Closing files can also include saving dataassociated with the open files into non-volatile memory 420.

In step 710, processor 418 stores any remaining but required informationfrom volatile memory (not shown in FIG. 4) to non-volatile memory 420.The processor 418 then enters a state where it does not accept anyadditional commands (step 712). This is because the external controller12 or an external charger 50 may be unaware that the IPG 100 isundergoing an emergency shutdown, and may continue to send commands andor instructions. Because the IPG 100 is being prepared for shutdown, anysuch commands or instructions are ignored. Processor 418 stays in step712 until the delay time T_(d) elapses and the processor 418 receivesthe reset RST signal 416 from the delay element 412 (step 714), whichsignal also opens switch 310 to disconnect ROD 312 from the power supply(e.g., battery 26). As shown in FIG. 6D, at this time, the power supplyvoltage at the ROD will begin to exponentially decay, taking a timeT_(f) to complete, which time may be abrupt. However, because the ISR isdesigned to have completed during T_(d), the IPG 100 is unaffected bythis fall off in the power supply voltage.

In cases where the processor 418 has been rendered non-operational, theprocessor 418 will be incapable of executing the shutdown routinedescribed. In such cases, the ROD 312 is still disconnected from thepower supply, and the reset RST signal 416 is still given to theprocessor 418 after delay T_(d).

Magnet 300 can be removed from the proximity of the IPG 100 to allow theIPG 100 to return to normal operation. For example, when magnet 300 ismoved away from the IPG 100, reed switch 512 will open and disconnectthe output of the magnet sensor from V_(bat). As a result, the output ofthe magnet sensor 306, and ultimately interrupt signal 414, are bothpulled low indicating that an emergency shutdown state no longer exists.Note that at this time the ROD 312 is still disconnected from the powersupply. Once the interrupt signal 414 begins to transition low, thereset signal 416 to the processor 418 will also go low and switch 310closed after some delay through the delay element. (Note that thehigh-to-low transition may be differently delayed from the low-to-hightransition delay of Td if the circuitry within the delay element 412 isnot balanced). Processor 418 and the ROD 312 may then enterinitialization and restore normal operation of the IPG 100.

Although the improved emergency shutdown circuitry is disclosed as beingactivated upon receipt of a magnetic field from a simple magnet,applications of the technique are not so limited. Instead, the sensorcan generically sense any shutdown signal wirelessly communicated fromany source external to the implantable medical device. For example, theshutdown signal may comprise a shutdown command telemetered to thesensor 306 from the external controller 12 or external charger 50, withthe sensor 306 in this case comprising a coil or other antenna. Even ifsuch relatively-sophisticated means are used to signal the IPG toshutdown, the improved circuitry 402 can still operate to shutdown theIPG in a controlled and graceful manner.

Although particular embodiments of the present invention have been shownand described, it should be understood that the above discussion is notintended to limit the present invention to these embodiments. It will beobvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Thus, the present invention is intended to coveralternatives, modifications, and equivalents that may fall within thespirit and scope of the present invention as defined by the claims.

What is claimed is:
 1. An implantable medical device, comprising: sensorcircuitry configured to produce a sensor output upon wirelesslydetecting an external signal; delay circuitry configured to produce adelayed output a delay time after the sensor output is produced; implantcircuitry configured to perform an action upon receipt of the sensoroutput, wherein the action is configured to be completed within thedelay time; and a switch configured to disconnect a power supply fromthe implant circuitry upon receipt of the delayed output.
 2. Theimplantable medical device of claim 1, wherein the external signalcomprises a magnetic field produced by a magnet.
 3. The implantablemedical device of claim 1, wherein the external signal comprises atelemetry signal.
 4. The implantable medical device of claim 1, whereinthe action comprises stopping therapeutic operation of the implantablemedical device.
 5. The implantable medical device of claim 1, whereinthe action comprises initiating a shutdown routine.
 6. The implantablemedical device of claim 1, wherein the action comprises closing open oractive files in the implant circuitry.
 7. The implantable medical deviceof claim 1, wherein the implant circuitry comprises a counter, andwherein the action comprises resetting the counter.
 8. The implantablemedical device of claim 1, wherein implant circuitry is furtherconfigured to perform operations, and wherein the action comprisesstopping at least some of operations being performed in the implantcircuitry.
 9. The implantable medical device of claim 8, wherein one ofthe stopped operations comprises receiving commands from an externalcontroller configured to communicating with the implantable medicaldevice.
 10. The implantable medical device of claim 1, wherein theimplant circuitry comprises a non-volatile memory, and wherein theaction comprises writing data to the non-volatile memory.
 11. Theimplantable medical device of claim 10, wherein the data comprises dataregarding the status of the implantable medical device.
 12. Theimplantable medical device of claim 10, wherein the data comprises atleast one time stamp.
 13. The implantable medical device of claim 10,wherein the data comprises an indication of an emergency shutdown. 14.The implantable medical device of claim 1, wherein the sensor circuitrycomprises conditioning circuitry configured to condition the sensoroutput.
 15. The implantable medical device of claim 1, wherein the powersupply comprises a battery.
 16. The implantable medical device of claim1, wherein the delay time is fixed.
 17. The implantable medical deviceof claim 1, wherein the delay circuitry comprises a capacitor configuredto set the delay time.
 18. The implantable medical device of claim 1,wherein the implant circuitry comprises a processor.
 19. The implantablemedical device of claim 1, wherein the implant circuitry comprises astimulation circuit configured to stimulate a patient's tissue.